In the area of adjustable frequency drives for alternating current induction motors, the driving of the motors at frequencies above the rated 60 hertz frequency of the motors is desirable to increase productivity of the motors and driven equipment. However, at higher frequencies, the rated torque and rated breakdown torque of the motor decrease significantly. Such lower torque levels are of particular importance in driven equipment where loss of load control can result in facility damage and injury to personnel.
Various provisions have consequently been developed to protect against inadequate torque levels when running motors at speeds in excess of 60 hertz. However, one of the areas for which there has been little development is the efficient and safe operation during deceleration of alternating current motors from speeds above 60 hertz down to 60 hertz. Presently, alternating current motors operating above 60 hertz are decelerated at a rate at which the torque required by the load will not be greater than the lowest torque level available during the deceleration down to a speed of 60 hertz. This torque level will be the rated breakdown torque of the motor at its highest speed at the initiation of deceleration. This, of course, is the lowest rated breakdown torque level during deceleration. Decelerating the motor at a rate that does not require a load controlling torque greater than the lowest rated breakdown torque level will be the slowest deceleration rate possible. Obviously, decelerating the motor in this manner is not particularly productive.